analysis of a pilot-scale anaerobic baffled reactor treating domestic ...

6 DISCUSSION: IMPLICATIONS FOR DESIGN AND OPERATION
“Reports that say that something hasn't happened are always interesting to me, because as we
know, there are known knowns; there are things we know we know. We also know there are
known unknowns; that is to say we know there are some things we do not know. But there are
also unknown unknowns - the ones we don't know we don't know” – Donald Rumsfeld
The purpose of this study was to investigate the performance of the pilot-scale ABR to (1) understand
the mechanisms of treatment of domestic wastewater; (2) identify potential advantages and
disadvantages of the reactor design for this application; (3) identify critical parameters for effective
design of an ABR for domestic wastewater treatment; and (4) to develop a dynamic biochemical
model of the pilot ABR to assist with design. It was hypothesised that the critical design parameter
was A-HRT and that phase separation would ensure stability of operation; consequently the pilot-scale
ABR was designed to have variable feed flow rates.
This chapter considers the experimental results in terms of the four objectives of this study.
6.1 PILOT-SCALE ABR STUDY – IMPLICATIONS FOR DESIGN
Apart from generally presenting and interpreting data describing performance of the pilot-scale ABR
during treatment of domestic wastewater at two municipal WWTPs, this study has in particular
developed an understanding of specific mechanisms of treatment that control start-up, sludge build-up,
solids retention and microbial community dynamics. In this section, three issues that affect design and
applicability of the technology, vis.
• pH buffering and phase separation
• Sludge accumulation and microbial community dynamics
• Pathogen removal and effluent characteristics
Analysis of these topics fulfils the requirement of the first objective of the study i.e. to understand the
mechanisms of treatment in an ABR treating domestic sewerage.
6.1.1 pH buffering and phase separation
Two important and interrelated measures of the condition of anaerobic digestion are the pH value and
the alkalinity concentration. The former affects microbial metabolic rates and the speciation chemistry
of several liquid-phase components and the latter affects the system’s ability to resist change in pH
value.
6.1.1.1 pH value
An important observation was that for most of the experimental study, the pH value measured in the
outlet stream was less than the pH value of the inlet stream of the pilot-scale ABR. This was initially
surprising since in many applications, anaerobic degradation of a waste stream results in the
production of significant amounts of alkalinity and free and saline ammonia, and a net increase of pH
is observed. However, examination of the literature showed that the effect was not unprecedented;
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